Hydro-morphodynamics triggered by extreme riverine floods in a mega fluvial-tidal delta

Maintaining accretion and progradation in a mega delta is crucial to its geomorphic stability and ecology. Extreme riverine floods can disturb hydro-sediment dynamics with great damage to the deltaic landscape, as for instance deltaic erosion. Nowadays, most mega deltas suffer from sediment starvation. Understanding the impact of extreme floods is a priority to determine the long-term fate of deltaic systems. Herein, we used the Delft 3D model and field data to study the hydraulics and morphodynamics of the 2016 extreme riverine floods in the South Passage (SP) of the Yangtze Delta. Results reveal that extreme floods can increase water levels, velocities, and bed shear stresses in an inner estuarine channel and mouth bar, while the flood has a weak effect in offshore areas. High-energy floods trigger strong tidal asymmetry and Euler residual currents, which intensifies downstream suspended sediment transport and bottom riverbed erosion. In comparison with those during extreme floods in 2016, net erosion after floods passed away was generated with seaward weakened magnitudes, the corresponding mean bathymetric erosion thickness was 19.97 cm, 12.71 cm and 4.62 cm in inner estuarine channel, mouth bar and offshore area, respectively. Even though the seaward deposition patches were due to lower scouring effect and converged sediment. Hydrodynamic increments in deeper channels were more significant, while shoals and deeper areas were strongly eroded with the lowest erosion between -5 m to -6 m isobath. These results further clarified the bathymetric patterns with highlights of extreme riverine floods that can amplify the sediment-insufficient risks in such mega fluvial-tidal delta.